The present invention relates to the control of electrical machines. The invention is primarily targeted at salient permanent magnet sinusoidal motors (PMSMs) with low per-unit mutual inductance. However, it is applicable to any type of sinusoidal machine (including induction motors and synchronous reluctance motors) that have self and mutual inductances that vary with rotor position.
Various methods are known for determining the rotational position of an electrical machine without using a dedicated position sensor. These are known as sensorless techniques. Many known methods use current sensors arranged to measure the current in the coils of the machine together with algorithms that can determine the position of the motor from the current measurements. Examples of such systems are described, for example in WO2004/023639A and WO2006/037966.
The present invention provides a control system for an AC electric machine, such as a motor, which comprises a rotor and a stator and a plurality of phase windings connected in a star formation, each winding having one end connected to a common neutral point and another end arranged to have a terminal voltage applied to it. The control system comprises switching means which may be arranged to control the terminal voltages applied to the windings, and control means, which may be arranged to control the switching means, for example so as to switch it between a plurality of states in each of a sequence of PWM periods. The control means may be further arranged to measure the voltage at the neutral point at sample times, which may for example be within the PWM periods, and to generate from the measured voltages an estimation of the rotational position of the rotor.
The neutral point voltage may include a component that varies with the inductance of the windings, and therefore with the position of the rotor. It may also include other components, such as a DC component and harmonics. In some cases, for example where this component that varies with position is small, the control means may be arranged to perform one or more steps to extract this component from the measured neutral point voltage and use this extracted component of the measured voltage to estimate the motor position.
The control means may be further arranged to measure a DC link voltage and to further use the DC link measurement to generate the position estimation.
The control means may be arranged to measure the neutral point voltage when all of the phases are connected to the DC link voltage thereby by to generate an estimation of the DC link voltage, and to use that estimation to generate the position estimation.
The control means may be arranged to compare the measured DC link voltage with the estimated DC link voltage and if they fail to meet a consistency condition, to generate a fault indication.
The sample times may be within the PWM periods. However in some cases they may be outside the normal torque-generating PWM periods during times when voltages are applied to the windings specifically for the purpose of position estimation.
The control means may be arranged to measure the neutral point voltage during two complementary states within one PWM period, during two pairs of two complementary states within one PWM period, or during each of a number of non-complementary states equal to the number of phases of the motor, within one PWM period. Where the machine is a three-phase machine this will be three active states.
The control means may be arranged to define a minimum state time for neutral point voltage measurement and a minimum number of active states within a PWM period in which the neutral point voltage needs to be measured. It may be arranged to determine a demanded net voltage for the PWM period, and to control the switching means so as to provide at least the minimum number of active states, each for at least the minimum state time. It may also be arranged to control the switching pattern so as to provide the demanded net voltage.
The present invention further provides a method of sensing the position of a sinusoidal machine comprising measuring a neutral-point voltage in at least two inverter states and determining motor position from the measured voltages.
The present invention is best suited to motors with low per-unit mutual inductance, but may also be suitable for motors with higher mutual inductance. It is therefore particularly well suited for motors with concentrated windings as these tend to have lower mutual inductance than distributed windings. It is equally applicable to systems with a single current sensor in the DC link and with multiple phase current sensors. It is suitable for any drive application but is particularly attractive for sensorless electric drive applications where low acoustic noise is required. Some embodiments are in particular suitable for use with PWM injection sensorless control.
The original target application for the present invention is a salient PMSM with concentrated windings and low per-unit mutual inductance with a single current-sensor drive for an automotive application.
Some embodiments of the present invention differ from other techniques that attempt to derive the electrical position from the neutral point voltage in that the resolution is far greater, resolving position down to levels associated with carrier frequency injection techniques, and is only limited by the resolution of the measurement hardware.
An example of an existing technique which uses the neutral point voltage is the ELMOS VirtuHall product which produces an electrical position signal with a resolution of 30° (electrical).
The present invention further provides a control system for an AC electric motor which comprises a rotor and a stator and a plurality of phase windings connected in a star formation, each winding having one end connected to a common neutral point and another end arranged to have a terminal voltage applied to it, the control system comprising switching means arranged to control the terminal voltages applied to the windings and control means arranged to control the switching means so as to switch it between a plurality of states in each of a sequence of PWM periods, wherein the control means is further arranged to measure the voltage at the neutral point when all of the phases are connected to the DC link voltage thereby by to generate an estimation of the DC link voltage.
The control means may be further arranged to compare a measured DC link voltage with the estimated DC link voltage and if they fail to meet a consistency condition, to generate a fault indication.
Other advantages of this invention will become apparent to those skilled in the art from the following detailed description of the preferred embodiments, when read in light of the accompanying drawings.